Panel dial switching system



Jan. 12, 1932. c. 1.. GOODRUM PANEL DIAL SWITCHING'SYSTEM Filed Sept.10, 1930 8 Sheets-Sheet 1 INVENTOR C. L GOOD/PUM ATTORNEY Jan. 12,1932.- c. L. GOODRUM PANEL DIAL SWITCHING SYSTEM Filed Sept. 10, 1930 8sheetssheet 2 wi mml g INVENTOR C. L. GOODRUM Jan. 12, 1932.

C. L. GOODRUM PANEL DIAL SWITCHING SYSTEM Filed Sept. 10, 1930 8Sheets-Sheet 5 Li H1 H1 INVENTOR C.L.GO0DRUM &

ATTOPNE Y Jan. 12, 1932. c. GOODRUM PANEL DIAL SWITCHING SYSTEM FiledSept. 10, 1930 8 Sheets-Sheet 4 IN l/E N TOR C. GOODRUM A TTORNEK Jan.12, 1932. c. L. GOODRUM PANEL DIAL SWITCHING SYSTEM Filed Sept. 10, 19308 Sheets-Sheet 5 INVENTOR C.L .GOODRUM A TTORNE V Jan. 12,1932. c.GOODRUM 1,840,357

PANEL DIAL SWITCHING SYSTEM Filed Sept. 10,. 1930 8 Sheets-Sheet 6 //vVENTOR C. L GOODRUM BY A TTORNE y Jan. 12, 1932. c. 1.. GOODRUM PANELDIAL SWITCHING SYSTEM Filed Sept. 10, 1930 8 Sheets-Sheet 7 c. L.GOODRUM 5. 66%

A TTORNE Y Patented Jan. 12, 1932 nmrso STATES PATENT OFFICE CHARLES L.GOODRUM, OF NEW YORK, N. Y., ASSIGNOR 'ro BELL TELEPHONE LABORA- ronins,INCORPORA E-n, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK PANEL DIALSWITCHING SYSTEM Application riled September 10, 1930. Serial No.480,852.

This invention relates to automatic telephone exchange systems and moreparticularly to a system in which link circuits are used to connect linefinders with idle senders.

The objects of the invention are to provide in a system of theforegoingcharacter a link circuit of the same mechanical structure as linefinders and other line extending switchesof the system, therebysimplifying manufacture, installation and maintenance, and also toarrange the distribution of links in respect to line finders terminatingon panel selector frames Over which said links hunt so as to increase.the efliciency of the links.

The link circuit contemplates in this invention is in part disclosed bymy copending application, Serial No. 466073, filed July 7, 1930, andcomprises a line finder selector and a sender selector for automaticallyconnecting a calling. line finder with an idle sender The link,therefore, being essentially a connecting circuit whose holding time issmall 7 compared to that of the line finder, is most 'efliciently usedif, after completing one association between a calling line finder andsender is immediately taken into use again to establish, in succession,similar associa tions between other calling line finders and idlesenders.

It is well known that the most efiicient relation betwen the size of theline finder group and the size of a cooperating link group-is that inwhich a suilicient number of calling line finders are always availableto keep the link in constant operation. To accomplish this purpose it iscustomary to provide groups of line finders which are large bycomparison to the cooperating group of links since it has been found inpractice that such an arrangement brings about the most desirable degreeof efiiciency in operation and economy in respect to the number of linkssupplied. One embodiment of the present invention has to do particularlywith the application of this principle to panel systems where the use oflarge contact banks permits the concentration of a large number of linefinders on a number of panel banks. It is understood, however, that theinvention is not limited to panel systems and can be easily modified foradaptationto other types of apparatus well known to the telephone art.In accordance with theinvention, therefore, one specific embodiment ofwhich is disclosed herein by way of illustration, this is accom plishedin the following improved manner:

The subscribers lines in an Ofiice are divided into major groupssuitable to the number which can be accommodated on the line finderframe or a group of line finder frames containing 50 line findersarranged and distributed as shown in my copending application abovereferred to and capable of handling the traffic originated by thesubscribers whose lines terminate at the frame. These groups ofsubscribers are divided into major subgroups and the major subgroup, inturn, subdivided into five minor subgroups whose lines are wired to thecontacts of'the finder banks.

It may further be considered that the line finder switches are providedwith as many sets of brushes as there are contact banks in the frame andeach set of brushes has access to the major subgroup of subscribers inthe bank with which the brush in question can operatively cooperate.lVith the arrangement of tripping the various line finder brushes in themanner disclosed in my copending application above referred to, it isonly necessary for the line finder to move a distance suilicient toengage the trip lever for each brushcooperating with the line bank ofcontacts in which the calling line appears to make contact with. anycalling line in any of the'five minor subgroups wired to the contacts ofthe bank. That is, when a subscriber initiates a call the particular setof brushes of an idle line finder having access'tothe group containinghis line is tripped and group in search of the calling line.

Conforming with the subdivisionof the subscribers group into main unitscapable of being served by'a, group of line finders, the line findersoffour such subscriber units are terminated; on vpanel banks with whichthe link finders of the links contemplated in this invention canoperatively cooperate. Four multiple banks are used, two for each 100line finders giving the link group an accessible capacity of 200 .linefinders. The link group itself consists of '60 elevators which arestandard with respect to a panel frame, 30cooperating when the contactbanks on one sideof the frame and 30 on the other.

The connection'between a calling line finder and a sender through themedium of a connecting link requires the use of six conductors so thateach line finder appearing on the link finder banks is wired to sixcontacts of the banks, three contacts on one bank and threecorresponding contacts on the second bank. The link finderis providedwith four sets of brushes, each brush having access to 100 terminals ofthe bank with which the brush set can cooperate. Since each line finderappears on two banks, two sets of brushes are tripped for eachconnection, either the lower two for connection to line findersterminating in the two lowerbanks or the upper two for line findersterminating on the two upper banks. A trip rod and trip magnetindividual to each link finder is used and, when themagnet is normal,the trip rod trips the two lower brush sets and when the trip mag-- netis energized, the two upper brush sets are tripped.

It is apparent that before a calling sub scriber can proceed withdialing the wanted number, the line finder connected to the calling linemust also be connected to an idle sender, which last function, in thepresent invention, takes place through the medium of the link. Since thelink circuit is common to a group of line finders and a group ofsenders, it is necessary for the link, when taken into use, to find thecalling line finder and seize an idle sender. The link finder elevatorof the link takeninto use, must, there fore, hunt for the calling linefinder and since the terminals of such a line finder may be anywhere ina pair of one hundred contact banks to which the link finder has-access,

considerable delay may result if, say, the link finder must travel overone hundred terminals before connecting with the calling line finder.Under such circumstances there is danger that the subscriber will beginto dial before .the sender is connected to the line and ready to receivedial pulses. It is desirable, therefore, to make this hunting time, onthe average, as short as possible. 7

To reduce this hunting time to approximately what it takes a line finderin my 00- third choice to the third group and so on up w to the tenthgroup for which the link finders of the last group are last choice. Thebanks of the frame are divided into ten vertical sections and the minorsubgroups of line finders appearing in the first-section of the bank areprogressively slippedthroughout the remaining nine sections in each ofwhich sections appear a group of link finder switches above referred toso that the minor subgroups of line finders whichare first choice in onesection of the bank for one group of links is second choice in anothersection for another group of links and so on throughout the entire tensections; the progressive slipping permitting each minor subgroup of tenline finders to appear as first choice in one of the ten sections of thebank. Normally, therefore, when a call originates in one of the linefinder frames and a'line finder is taken into use, a link is chosen, ifone is available, from the group of links in the section where the linefinder minor subgroup containing the calling line finder appears asfirst choice.

As each link of the group is taken into use, a pair of contacts of achain circuit is closed which is completed when the last ofthe six linksof the group is made busy. When, now, another call is initiated in theminor subgroup of line finders which are first choice to the group oflinks in which all links are busy, the above-mentioned chain circuitswitches the link start conductor to the group of links appearing inthatv section of the frame where the line finder group containing thecalling line finder appears assecond choice. A similar operation takesplace in switching a calling line finder to the succeeding preferencechoices should all of the links of the previous choices be busy. If afirst choice link is chosen, the associated link finder will then haveto hunt only a maximum of ten terminals in order to locate the linefinder taken into use by the calling subscriber. It is possible,however, that during heavy trafiic when the links are busy, tenth choicelinks may be taken into use, in which case the link finder may have tohunt over a maximum of one hundred terminals before locating the linefinder associated with the calling subscribers line. V

1 In general, the system covered by the present invention, taken inconnection with the invention covered by. my above-mentioned 'copendingapplication, Ser. Bio-466,073, filed July 7, 1930, functions in thefollowing manner:- hen subscriber in one of the subgroups initiates acallsnd a line finder of a group having access to the group of thecalling line is taken into use, the line finder proceeds to connect withthe terminals of the calling line and, at the same time, extendsstarting ground to a link start circuit which. normally controls thegroup of links for which the line-finder minor subgroup containing theline finder taken into use is a first choice and, also, the group ofinks for which it is second choice. Since the line finder subgroups arearranged to appear in the link finder frame in such a mannerthat theline finder minor subgroup appearing as second choice in one section oi.he bank appear as a first choice in another section of the banks, it isevident that the same start circuit can control two separate groups oflinks. each group being first choice to the lowest line finder minorsubgroup in the section where the link group appears. Since, also, theline finder minor subgroups on-the two upper banks on the line finderframe'are arranged symmetrically with respect to the line finder groupsin the two lower banks, the same start circuit likewise controls thesame groups of links for cooperation with the line finder minorsubgroups on the two upper banks.

Each start circuit consists of a group of relays and a cooperatingcircuit while the link circuit consists of a group of relays, a linkfinder and sender selector elevator together with associated brush sets,commutators and magnet equipment normally formthe component parts of apanel selector. The start conductor of a link start circuit is connectedto the first link of the group; each link of the group advancing thestart conductor to the next succeeding link by connecting the start wireof the previous link over a commutator segment and brush of thesucceeding link to the start wire of the said succeeding link. As soonas the selected ll link moves off normal the commutator connection ofthe selected link with the start conductor is broken. so that anyfurther electrical condition on the start circuit will no longerv affectt ie selected link. but the start conductor is then connected through anoil'- group to indicate that the link is busy and starts still othercircuits for controlling the movement of the sender selector elevator tohunt for an idle sender.

As already noted, the entire groups of links accessible to two hundredline finders is divided into small groups of six, each of which is firstchoice to one major subgroup of twenty line finders. The link startcircuits are arranged with reference to each other so that theprogressive advancement of the starting conductor from one link startcircuit to the next is controlled by a relay in each start circuit whichis responsive to the chain circuit above referred to of the links of thegroup, so that a calling line finder may be switched successively fromone group of links to the next in the order of preference which eachgroup of links has for the calling line finder major subgroup appearingat that section of the frame where the available link group is located.This operation of advancing from one group of links to the next throughthe medium of successive start circuits continues until an idle link inone of the groups is found and taken into use whereupon the associatedlink finder will proceed to hunt f or the calling line finder.

Each link is provided with a relay which, in cooperation. with a relayin the start circuit, controls the operation of the tripmagnet of thelink finder. The operation or non-operation of the relay in the linkstart circuit determines which of the pair of brushes of the link finderis to be tripped. This determination is made in the link group startcircuit where a separate start ing conductor is provided for each of thetwo minor subgroups of line finders, namely, those appearing on the twoupper banks and those appearing on the two lower banks of the frame.Those appearing on the two upper banks are each connected to a separatestart conductor which, when grounded by the calling line finderconnected thereto, controls circuits for operating the trip magnotcontrolling the tripping of the two upper brush sets. Those link findersappearing on the two lower banks are each connected to other separatestart conductors which, when grounded by the calling line finderconnected thereto, cause the selection of an available link in the groupbut do not effect the trip magnet circuit. Under such circumstances, thetrip magnet is not operated since the calling line finder is located inthe two lower banks and, according to the mechanical structure of thetrip mechanism, the two lower brush sets are tripped.

When the link finder has connected with the calling line finder and thesender selector has seized an idle sender, the conductors connecting theline finder with the sender are closed through. Dial tone is thereaftertransmitted to the calling subscriber in the Well known manner and thesender functions to control the setting of the selectors to the calledsubscribers line.

When this function. is completed, circuits are closed in the sender fromthe line finder which serve to disconnect the link finder switch fromthe ter inals of the calling'line finder and cause it to return tonormal. The relay connecting the six conductors together is thereafterreleases and the link circuit is thus restored.

A clearer conception of the scope and purpose of the invention will beobtained by a consideration of the following description taken inconnection with the attached drawings in which:

Fig. 1 shows a subscribers group circuit;

Figs. 2 and 3 show a combined line finder and district selector;

Fig.4 shows two link group start circuits;

Fig. 5 shows a link circuit;

Fig. 6 is a schematic drawing of a sender;

Fig. 7 shows schematically the arrangement of the link finder elevatorson a link finder frame together with the distribution of the line findergroups thereon;

Fig. 8 shows schematically the cooperating relationship between the linkstart circuits of the type shown in Fig. 4 and the link finder frameshown in Fig. 7;

Fig. 9 shows the manner in which Figs. 1 to 6 inclusive are to bearranged with respect'to each other for a complete description andunderstanding of the present embodiment of the invention;

Fig. 10 shows the manner in which Figs. 7 and 8 are'to be arranged withrespect to each other in order to show clearly the interconnectionbetween line finder groups and the link start circuits.

It is to be noted that Fig. 5 is a skeletonized disclosure of the sendershown in U. S. Patent 1,589,940, granted to O. H. Kopp, June 22, 1926,and as modified by Patent 1,690,206 granted to A. Ransford on Novemher6, 1928. Reference to both patents is made for operations not completelyclescribed herein.

l/Vhen a call is initiated by a subscriber in a particular group, suchas subscriber of substation #100, for example, by removing his receiverfrom the switchhock, a circuit is closed for relay 124 extending frombattery through resistance 125, winding of relay 124, inner contacts ofrelay 101, over the subscribers loop to ground at the outer contacts ofrelay 101. Relay 124 operates and closes a circuit fromground on outercontacts, conductor 126, No. 1 back contacts of relay 128, back contactsof relay 102, winding of group start relay 116 to battery. Relay 116operates inthis circuit and connects ground on conductor 130 over itsleft contacts to conductor 123 which extends to the marking segment ofall commutators of the line finders serving the particular verticalsection of the line finder frame in which the calling line is located asfirst preference. It also extends the same ground on conductor 130 overthe right contacts of relay 116 to conductor 111, left outer backcontacts of relay 213, conductor 251, conductor 332, left upper contactand cam 305 in position 1, cam 317 and right upper contact thereof,conductor 331, left windingof relay 206 to battery. Relay 206 operatesand closes a circuit for sequence switch magnet 300 from ground on cam320 and the right lower contact thereof conductor 328, right inner backcontacts of relay 205, right inner front'contacts of relay 206,conductor 324, left upper contact of cam 319, winding of magnet 300 tobattery. Switch 300 operates and advances to position 2. lVhen theswitch has advancedbeyond position 1 the operating circuit for relay 206is opened at the left upper contact of cam 305 but a holding circuitexists for this relay over its left winding and left outer frontcontacts, conductor 254, lower contacts of cam 308 to ground. When theswitch reaches position 2, a circuit is closed for relay 205 from theafore traced. ground on conductor 251, left contacts of cam 301,conductor 321, conductor 23?, winding of relay 205 to battery. Theground on conductor 321 is further extended in parallel through theright outer back contacts of relay 213 to the winding of relay 212 tobattery. Both relays 212 and 205 operate in their respective circuits.The operation of relay 212 causes its back contacts" to be broken butotherwise performs no useful function at this time. The operation ofrelay 205, however, closes a circuit for the updrive magnet 215 fromground on cam 320 and the right lower contact thereof, conductor 328,right inner front contacts of relay 205, conductor 248, cam 303 and leftupper contact thereof, conductor 329, left outer contacts of relay 205,conductor 247, right inner contacts of relay 213, winding of line finderupdrive magnet 215 to battery. The elevator will now be raised and, assoon as brush 229 makes contact with the first marking terminal to whichconductor 123 is connected, a circuit for relay 213 is closed from theaforetraced ground on conductor 123. brush 229, left normally madecontacts of relay 213, winding of relay 213 to battery.

Relay'213 operates, locks via conductor 233,

line.

tacts of relay 202, conductor 249, upper contacts of cam 316, conductor238, left inner contacts of relay 205, winding of relay 205 to battery.After relay 213 has operated as above described, a circuit is closed forthe trip magnet 214 from battery through the winding of trip magnet 214,right outer front contacts of relay 213, to the aforetraced ground onconductor 321'.

When relay 213 operates, the circuit for relay 212 is opened at itsright outer back contacts Relay 212 is made slow to release and will,therefore, close its contacts sometime after trip magnet 214 isoperated. Vhen, however, its back contacts are made, a circuit is againclosed for the updrive magnet 215,this time through the contacts ofrelay 212, to conductor 247 connecting with the aforetraced magnetoperating circuit to ground on cam 320. The line finder is now movedupwardly and trips the set of brushes engaged by the trip finger andthereafter proceeds to hunt for the calling line in the subscribergroup. hen the terminals of the calling line are eni countered, acircuit is completed from battery through resistance 109, inner contactsof relay 124, conductor 114 and associated terminal, brush 238,conductor 220, winding of relay 202, conduct-or 235, right contacts ofcam 308 to ground. Relay 202 operates in this circuit and, at its leftback contacts, opens the holding circuit of relay 205 to brush 241. Thisrelay, however, does not release immediately it also has anotherholdingcircuitby way of "-7 ground through brush 221, centering commutatorstrip 232, conductor 231, conductor 249, upper contacts of cam 316,conductor 238, left inner contacts of relay 205, winding of relay205 tobattery. Relay 205 remains operated until the updrive magnet 215 hasdriven the elevator to the centering position of the calling lineterminals whereupon an insulating segment is encountered on thecentering commutator,the locking circuit is opened thereat and relay 205is then released.

When relay 202 operates a circuit is closed for relay 209 from ground onthe right contacts of relay 202, right normally madeconof relay 209,right winding of relay 209 to battery. Relay 209 operates and locksthrough its right make contacts, conductor commutator segment 236, toground on irush 234. Relay 209, over its left middle contacts, advancesthe start conductor 111 to t next idle line finder in the preferencesubgroup for succeeding calls and, on its left outer contacts, closestwo contacts of a series chain for o erating master transfer reay 128,both of which functions are fully exiled in my copending applicationabove red to.

ii in? release of relay 205 opens the circuit of updrive magnet 215 andstops the line finder on the terminals of the calling subscribers t alsoconnects ground from cam 304 202 will release when battery throughresistan ce 109 is disconnected from conductor 114 by the release ofrelay 124. Should it come to pass, however, that when the line finderconnects with the calling line and the aforetraced circuit for relay 101is open because of some trouble condition and relay 101 fails tooperate, then relay 202 will remain operated and close on its frontcontacts a parallel circuit from an aforetraced ground on conductor 223,left front contacts of relay 202, low

resistance 240 to the winding of relay 202.

This circuit, in parallel with the one through the winding of relay 202,imposes a low potential on the line terminal connected to conductor 114,marking the terminal as busy by shunting the circuit of relay 202 of anyother passing line finder in search of a calling line, therebypreventing the false operation of relay 202 of this other line finderand the consequent false stopping of its associated line finder elevatoron the terminals of the line with a trouble condition on the circuitpath of relay 101. The connection to ground with conductor 110 andassociated terminal by way of conductor 219, resistance 242, lowormiddle back contacts of relay 200, cam 304 1 and right upper contactthereof to ground marks terminal 127 and other multiple terminals ofcalling line 100 in the terminal bank of final selectors with a reducedpotential to render the line busy to hunting final selectors.

lVhen the line finder elevator moves off normal ground is connectedthrough brush 234, commutator segment 236, conductor 233, back contactsof relay 207, conductor 409, left outer back contacts of relay 406,right inner contacts of relay 404, left back contacts of relay 402,conductor 410, commutator segment 500 of the link finder circuit,brushes 501 and 502 in the normal position of the link finder elevator,commutator segment 504, conductor 510, bottom No. 1 back contacts ofrelay 505, top back contacts of relay 512, winding of relay 506 andbattery, and in parallel through the bottom contacts of relay 516,winding of relay 507 to battery. Both relays 506 and 507 operate andperform functions which will be hereinafter described. In the meanwhilethe release of relay 205 closes a circuit from battery through thewinding of sequence switch magnet 300, cam

319 and the left upperconta'ct thereof, conductor 824, right inner frontcontacts of relay 206, right inner back contacts of relay205, conductor328, right lower contact of cam 320 to ground. Sequence switch 300 isadvanced in this circuit to position 3 and relay 206 is unlocked at theleft lower contact of cam 2308. Switch 300will remain in position 3until a signal is received from a sender connected to the line finderthru the medium of the link shown in Fig. 5 and which signal is to theeffect that the line finder circuit may begin to extend the connectionto the distant office under the control of operated registers in thesender.

Figs. 7 and 8 show the relationship, in a schematic manner, between thedistribution of the line finders on a link finder frame, shown in Fig. 7with respect to the trip rods of the link finder elevators on one sideof the finder groups the manner in which links are asslgned and also themanner In winch the link startlng conductors are advanced from one groupof links to another when the links of the preceding group are busy.

l 'InFig. 7 the ten vertical sections of the link finder bank of theframe are represented.

by 7 01, 702, etc. to 710 inclusive, and the line finder minor subgroupsby A1A2, B1B2, L3 L4, M3M4, etc. The letter designations of the linefinder minor subgroups represent the same minor subgroup and thenumerical sufiix associated therewith indicates the bank in which theminor subgroup appears. Sinceieachgroup appears in either the two lowerbanks or the two upper ones the letter designation is thesame for eachpair of banks. I

We will now consider the link start circuits shown in Fig.4 andparticularly the one to which start conductors 409, 411, 414 and 413 areconnected; Conductor 409 is the start conductor of the line finder minorsubgroup minor subgroup M3-M4 appearing insecbanks of the link finderframe. It is to be noted that while the minor subgroup Ale-A2 and L3-L4appear as first choice in section 7 01 they appear as'second choice insection '4" 10 and while minor subgroups B1-B2 and M3M4 appear as firstchoice in section 710 they are second choice in section 701. A furtherinspection of Fig. 7 also shows that each pair of sections of the frameare symmetrically reciprocal with respect to each other; that is, thefirst and second choice minor subgroups in one section are second andfirst choice minor subgroups in the reciprocal secend section. Forinstance, line finder minor subgroup C1C2 is the first choice group inthe two lower banks of section 7 02 and line finder minor subgroup D1D2is second choice in the same section of the bank while in section 709,minor subgroup C1-C2 is second choice and group Dl-DQ first choice. Aninspection ofthe drawings will show that the same relationship holdstrue for all other sections and minor subgroups in both pairs of banks.

- It will be observed from an inspection of 5 the operation of relay 507prepares a path from the winding of trip magnet 514 over the upper innerfront contacts of elay'507 to conductor 515 which extends to therightinner contacts of relay 401. Since, however, we have assumed that thestart ground was connected to conductor 409 associated with thelinefinder minor subgroup Ail- A2 in'the two lower banks, no circuit isavailable to operate relay 401 since,

as shown hereinafter, the circuit for this relay 1S controlled by aground on conductor 411. When ground is connected to conductor 409,therefore, the trip magnet circuit is not closed'at the time relayoperates. Under such circumstance trip rod 711 is not r0- tated and, dueto the mechanical structure of the trip fingers 714 and 715, the brushset associated with each of these trip fingers is tripped when theelevator moves upwardly and not the brush-set associated with the tripfingers '7 16 and 717 cooperating with the two upper banks. The linkfinder elevator, when operating, will therefore hunt for and engage tneterminals of the calling line finder in minor subgroup A1A2 in the twolower banks. On the other hand, if start ground is applied to conductor411 which is connected to the line finder minor subgroup L3L4terminating in the two upper banks a circuit is closed from ground onconductor 411, left inner bac; contacts of relay 406, right innercontacts of relay 405, right inner back contactsof relay 402, rightouter normally made contactsof relay 401, winding of relay 401 tobattery. Relay 401 operates and, on its left contacts grounds conductor410wl1ile on i s right inner contacts it grounds conductor 515 of thetrip magnet circuit above traced.

tion 710 as the first choice in the two upper l Vhen relay 507operates,therefore, a circuit is closed for trip magnet 514, from battery throughthe winding of trip magnet 514, top innerfront contacts of relay'507,conductor 515 to ground. Trip magnet 514 is operated and rotates triprod 711 to its proper posi tion for tripping the brush-sets associatedwith trip fingers 716 and 717 on the subsequent upward movement of thelink finder elevator. When the up-drive magnet 513 opcrates, the linkfinder, with its proper pair of brush sets released, will hunt for andengage the calling line finder in minor subgroup L3L4. The operation ofrelay 507 closes ground on conductor 570 which looks relay 401 undercontrol of relay 507.

Conductor 410 is connected with commutator strip 500 of the'first linkof the group and when the link finder of this first link is moved offnormal and brush 502 clears the commutator segment 504 the start circuitconnection to the chosen link is broken and when the link finder hasmoved upward so that commutator brush 502 connects with commutator strip503 conductor 410 is then extended to conductor 571 of the nextsucceeding link by the way of brushes 501 and 502, commutator segment500 and conductor 571. When the link associated with conductor 571 islikewise taken into use and the link finder elevator moved upward sothat its commutator brush 502 makes contact with its commutator segment500 the link start conductor 410 is then further extended to the thirdlink provided, of course, that the first link connected directly toconductor 410 is still busy or its link finder elevator is otherwise offnormal. This will hemore clearly apparent by reference to Fig. 5. Inthis manner successive links ofthe group are taken into use by the linkstart circuit which controls them. Also, when the link finder elevatormoves ofi' normal, brush 517 engages commutator segment 518 and closes acircuit for relay 543 from ground through brush 517, commutator segment518, conductor 519, winding of relay 543 to battery and, in parallel,through the bottom outer contacts of relay 508, to the winding of relay516 and battery. Relays 543 and 516 operate. Relay 543,.in conjunctionwith relay 507, which was released in the meanwhile as explainedhereinafter, closes a pair of contacts in a chain circuit of the linkgroup of the section through other pairs of contacts of relays 543 and507 of other busy links of the group to the grounded right contacts ofrelay 507 of the last link in the group. If, therefore, the whole groupof six links serving a section is made busy when the last remaining idlelink of the group is taken into use, a circuit is completed from groundon the back contacts of relay 507 of the last link through the chain ofpairs of contacts above mentioned, conductor 426, contacts of relays 507and 543 atend of chain, conductor 422, winding of relay 402 to batterycausing relay 402 to operate. Should, now, another call be initiated anda line finder of minor subgroup A1--A2 associated with conductor 409 betaken into use ground on this conductor is extended to conductor 423over its previously described path and by way of the left front contactsof relay 402. Conductor 423 is similar to conductor 410 except that itis the starting conductor of the group of links cooperating with thesection wherein the line finder minor subgroup ill-A2 is second choice.By reference to Fig. 7 it will be noted that this link group is locatedin section710 of the link finder frame. A link is chosen, therefore, inthis section to connect with the terminals of the calling line finder ofminor subgroup A1A2 and in which the link finder concerned hunts in thesecond group to locate the calling line finder.

It will be observed that if, during the time that all the link circuitsin section 701 are busy, a line finder in minor subgroup L3L4 in the twoupper banks is connected to a calling line, then conductor 411 will begrounded. Relay 402 being operated as a result of the all-busy conditionof the link group of section 701, a circuit is closed for relay 400 fromground on conductor 411, left inner back contacts of relay 406, rightinner back contacts of relay 405, right inner front contacts of relay402, left normally made contacts of relay 400, winding of relay 400 tobattery. Relay 400 operates, grounds conductor 423 on its right contactsand, on its left contact grounds conductor 412. Conductor 423 is thestart conductor for the link group cooperating with section 710 in whichsection line finder minor subgroup L3L4 appears as second choice, andconductor 412 controls the operation of the trip magnet and the chaincircuit through relays 507 and 543 V of the links. Trip fingers 719 and720 of the trip rod 718, to choose as an illustration the first link inthe group of section 710, would then trip the two upper brushesassociated therewith for connecting the link finder with the callingline finder in the second choice minor subgroup L3L4 in the two upperbanks of this section.

Since each line finder minor subgroup is -i. l .11 one of the tenvertical sections of th frame follows, as has already been shown, thatthe start" conductor of each of the line finder minor subgroups isconnected to that link start circuit which controls the link minorsubgroup in the section where the line finder group appears as first choce. It has also been shown, that a link start circuit not only controlsthe link group having first choice accessibility to the line finderminor subgroup appearing in the section where the links are located, butalso controls the link group in the section where the same line finderminor subgroup appear as second choice. Specifically, the link startcircuit shown to the left in Fig. 4, and connected to start conductors409, 411, 414 and 413 controls the link group of section 701 for the twoline finder minor subgroups of the firs choice therein, and the linkgroup of section 710 wherein the minor subgroups of first choice insection 701 appear as second choice. Remembering that the link finderframe is divided into pairs of sections, each reciprocal with the other,it follows that the start circuit above mentioned controls the linkgroup of section 701 as first choice for the line finder minor subgrouptherein appearing as first choice and the link group in section 710 assecond choice for the line finder minor subgroup therein appearing assecond choice but which group appears as first choice in section 01. Asa concrete illustration conductor 413 is associated with the firstchoice line finder minor subgroup B1B2 in the two lower banks of section710 and conductor 414 with the first choice minor subgroup M3M4 in thetwo upper banks of section 710. it will be noted that both of theseminor subgroups appear as second choice in section 701. l/Vhen,therefore, ground is connected to conductor 413 it is extended toconductor 423 by way of the right outer back contacts of right innercontacts of relay 408, left inner back contacts of relay 403, left outernormally made contacts of relay 400, winding of relay 400 to battery.Relay 400 operates, grounds conductor 423 and also conductor 412 which,as already described, controls the circuit of trip magnet of each linkfinder in section 710 for tripping the two upper sets of brushes. 7

lVh-en call originates in either of these groups and all of the links insection 710 are busy chain circuit, identical with the one desc, ibedabove in connection with links in section 7 O1, is closed therebyconnecting ground over conductor 425 to the winding of relay 403. groundon starting conductor 413 to conductor 410 which, as already shown, isthe start conductor for the groups of links in section 701, and connectsconductor 414 to the winding of relay 401. Then relay 401 operates theresult of a ground on conductor 414 it grounds conductor 410 and alsoconductor which latter conductor controls the trip of thelink group ofsection 701 as already described.

.As each pair of link groups becomes busy,

. link finder frame.

Relay 403 operates and extends the the starting conductors connected tothe as sociated link st-art circuit are successively transferred to thesucceeding start circuit of another pair of link groups; the callingline finder, in such cases, becoming connected to a link of a moreremote preference depending on the link st. rt circuit to which thestart conductor with which it is ultimately connected is transferred.This is apparent from an inspection of Figs. 7 and 8 which, together,show the relation between the live start circuits, two of which areshown in cetail and are identical with the start circuits shown in Fig.4, and the ten link groups of the If, for example, the link groups ofsection 701 and section- 710 are busy, then. relays 402'and 403 in thassociated separate chain circuit of the link groups in sections 7 01and 710 respectively; A circuit is now closed for relay 406, from groundat the left outer contacts of relay 403, right uter contacts of relay402, winding of relay 406 to battery, causing this relay to operate. If,during the period when relay 406 is operated, which is to say whenthe'link group in section 7 01 andthat in section 710 are busy and forwhich minor subgroups line finder groups A1A2 and L3L4 are first andsecond choices respectively, a call is initiated in minor sugroup A1A2,then the ground on conductor 409 will close circuit for relay 404 overthe'left outer front contacts of relay 406, operating relay 404 andlooking it over its left contact to thestart conductor 409. Shoulditcome to passthat, during this time, relay 406 is releaseddue to therelease of either relays 402 or 403 as a resu t of links becoming idleinthe respective link groups controlled bythese relays, then relays 404will remain locked without being in any way affected by the release ofrelay 406 and a link will be chosen from the group controlled by thestart circuit with which the start conductor has been connected. Relay404, at its right inner contact, breaks the extension of conductor 409to the contacts of relay 402, and, on its right outer contact, groundsconductor 415 which is connected to the left outer armature of relay 417of the next succeeding start circuit, that is, the start circuitcontrolling the pair oflink minor subgroups in the sections Where theline finder groups A'1A2 and L3-L4' appear as third and fourthpreferences respectively. In F 7 ie finder minor subgroups A1A2 and L3L4third and fourth preference in sections and 706 respectively and relay417 belongs to the start circuit controlling the link roups of sections705 and 706. This startcircuit is connected to the starting conductorsof line finder minor subgroups K1K2, T3T4 and U3U4 which are first andse ond choices, respectively, in the two sections 705 and 706 in thereciprocal sense in which choices have been described above inconnection with the other'line finder minor subgroups. Consequently theground connected to the contacts of relay 417 by the operation of relay404 of the previous start circuit becomes, in effect, the start groundfor the link start circuit controlling the link group in section 705.Ground on conductor 415 is new extended to conductor 416 by way of theleft outer back contacts of relay 417, right inner contacts of relay418, left back contacts of relay 419 to conductor 416. Conductor 416 isthe start conductor for the link group cooperating with section 7 05 anda link finder would thus be selected to locate the calling line finderin minor subgroup A1A2 in section 705. On the other hand, if all thelinks in section 705 are busy as well as those in the previous groupthen the closure'of the chain start circuit as previously described forthe links in section 701 would operate relay 419 and transfer the startground on conductor 415 through the left front contacts of relay 419 tostart conductor 424 of the link group in section 706. In this manner itcan be easily shown that the start conductors connected to any linkstart circuit are progressively advanced to the successive start circuits of other link groups having more re mote preferences to thecalling line finder minor subgroup than all of the previous link groupsto which the start conductors have been advanced. The operationcontinues un-- til connection is made with a start circuit of a pair oflink groups which have idle links, one of which can be connected to thecalling line finder.

It can likewise be easily shown that simultaneous calls in differentline finder groups can be handled by the link start circuit to whichsuch groups are connected. For example, if there were acalling linefinder in the group connected to conductor 409 and another calling linefinder in the group connected to conductor 411 the ground on conductor411 operates relay 401 as already described grounding in turn, the linkstart circuit conductor 410 and the trip magnet circuit conductor 515.The operation of relay 401 will cause the chosen'link in the group totrip its two upper brushes and connect with the'calling line finder inthe two upper banks; Ground on conductor 411 is thereafter removed, asexplained hereinafter, causing the release of relay 401, and theconsequent removal of ground from conductor 515. The calling line finderin the? minor subgroup controlled by the conductor 409, however, has notyet been connected to a link finder so that ground on conductor 410 isstill connected thereto from conductor 409 in spite of the release relay401, The presence of ground on conductor 410 will, of course, startanother link and the fact that there is no ground on conductor 515 willcause the lower set of brushes to be tripped for connection to theterminals of the calling line finder 0f the two lower banks.

Returning now to a continuation of the further operation of the selectedline finder shown in Figs. 2 and 3, it will be recalled that, as soon asthe line finder elevator moved off normal, a circuit was closed for linkrelays 506 and 507 causing both of these relays to operate. Relay 506closes a circuit for the up-drive magnet 513 from ground at the topcontacts of relay 508, front uppermost contact of relay 506, winding ofthe up-drive magnet 513 to battery. Relay 507 connects the winding ofthe associated trip magnet 514 through the top inner front contact ofrelay 507, conductor 515 to the right inner contact of relay 401. Theup-drive magnet 513 will new drive the link elevator in search of thecalling line finder with the trip magnet 513 energized or not dependingupon whether the calling line finder appears in the two upper banks orthe two lower banks of the link finder frame as already described.

If it is located in the two upper banks then right inner contacts itgrounds conductor 515 which closes the circuit of the trip magnet 414 asalready described. As soon as the link finder elevator has been drivenbeyond its normal position the operating circuit of relays 506 and 507is opened at commutator segment 504. Relay 506 is slow to release,however, and holds up until a circuit is closed for relay 516 fromground on brush 517, commutator strip 518, which is engaged by brush 517as soon as the elevator is 011' normal, conductor 519, bottom outercontacts of relay 508, winding of relay 516 to battery. Relay 516operates in this circuit, supplies locking ground for relay 506 throughits No. 2 contacts and supplies another ground for the up-drive magnet513 through its No. 1 contacts over the top outer contacts of relay 506.Relay 507 will release at the time the operating circuit is removed, butno harm is done since the tripping operation is completed before thisoccurs, and, with the operation of relay 516, its operating circuit isopened at another point, thereby preserving, in cooperation with thecontacts of relay 543, the pair of chain contacts of the link group busycircuit. The operation of relay 506 also connects the winding of relay508 to brush 520 by way of the top middle contact of relay 506. When theterminals of the calling line are encountered, relay 508 operates inseries with relay 207 in the following circuit: battery through thewinding of relay 508, top middle contacts of relay 506, brush 520,conductor 521 and associated terminal, right Winding of relay 207, topcontacts of jack 243,,conduct'or 255, left upper and right lowercontacts of cam 302 to ground. Relay 207 operates and looks over itsleft winding, conductor 233, commutator strip 236, brush 234 to groundand opens the start conductor 409. Relay 508 looks over the operatingcircuit through its bottom inner contacts directly to brush 520independent of the contacts of relay 506. The updrive magnet circuit isnot opened, however, as relay 516 is held locked to ground on thecentering commutator segment 522, conductor 560, top outer contactsof'relay 523, to the Winding of relay 516.

, When an insulating segment is encountered,

indicating thereby a propercentering of the elevator brushes on thecontacts with which they make, relay 516 releases, opens the circuit ofthe updrive magnet 513 and brings the link finder elevator to rest withthe scribed hereinafter. 40

sender, which is to say that the elevator does not restore to normalwhen a connection to a sender is terminated but only after it hassuccessively hunted to the top of the frame and returned to normal asmore fully de- When the link finder elevator moved 0E normal, and brush517 grounded commutator strip 518, a circuit was closed from groundonstrip. 518, conductor 519, bottom outer contacts of relay 523, No.

. 3 contacts of relay 505, top middle normally made contacts of relay527 ,top winding of relay 526 to battery. Relay 526 operates and, at itstop outer front contacts closes a circuit for relay 527 from ground onthe above men- I v. tioned contacts, top inner normally made contacts ofrelay 527, winding of relay 527 to battery. Relay 527 operates and locksto grounded conductor 519 by way of its top middle front contacts and,at the same time, opens the aforetraced operating circuit of relay 526.If the sender selector is connected at this time-to the contacts of anidle sender there will be no circuit to hold relay 526 because brush 569is in contact with an insulating segment of the centering commutator 531in the known manner, providing no holding circuit for the top windingand the locking circuit over itsbottom winding is connected to thebrush530 which is connected to battery. If, on the other hand, theelevator brushes are connected to the terminals of a Relay 506 is alsoreleased by the the calling line finder.

busy sender then ground on the bottom front contacts ofrelay 527 ofanother link operatively connected with the sender is present onterminal 524 and holds relay 526 operated over its lower winding.

Under such circumstances a circuit is closed for the updrive magnet 528from ground on the top outer front contacts of relay 526, top innerfront contacts of relay 527, winding of updrive magnet 528 to battery.The sender selector proceeds in search of an idle sender, relay 526being prevented from releasing by the ground circuit through thecentering coinmutator 531 and connecting conductor 555 to the top innercontacts andtop winding of relay 526. vVhen an idle sender isencountered, the presence of battery on the lower winding of relay 526and the absence of the holding circuit over its top winding through thecentering commutator causes relay 526 to release, opening the updrivemagnet circuit and causing the elevator to come torest with its brusheson the terminals of the selected sender. The release of relay 526 alsocloses a circuit for cut-through relay 525 from ground on the top outerback contacts of relay 526, top outer contacts of relay 527 No. 4contacts of relay 505, No. 4 contacts of relay 516, winding of relay 525to battery. Relay 525 operates and'connects the conductors leading tothe selected sender with the brushes of the link finder connected withthe terminals of The release of relay 526 connects the test brush 530through the bottom back contacts of relay 526 to ground on the bottomfront contacts of relay 527, thereby causing the sender seized by thelink to test busy to all other hunting sender selectors.

If all senders terminating in the upper portion of the bank of thesender finder test busy, then the sender finder elevator will continueits upward movement until the commutator brush 561 connects with thecommutator segments 554 and 562, whereupon a circuit is closed fromground on commutator segment 554, brush 561, commutator segment 562,conductor 557, winding of relay 505 to battery. Relay 505 operates inthis circuit and locks over conductor 559, commutator strip 551, toground on brush 563. Relay 526 releases since its lower test'winding isno longer connected to the sender testing terminals and its top windingis disconnected from the centering commutator, the updrive magnetcircuit is opened and relay 505 closes the circuit forthe downdrive'magnet 529 from ground on its No. 2 contacts, thereby restoringthe elei' ator to normal whereupon relay 505 releases and the originaloperating circuit for relay 526 is again closed and the above cycle ofoperations is repeated until an idle sender is found or starting groundis removed from conductor 410.

It should be noted that the sender selector commutator is provided withtwo segments 571 and 553 whose arrangement with respect to the senderselector brushes 565, 566, 564, 567 and 568 is such that when thesebrushes are connected to the terminals of the tenth sender, countingfrom the top of the bank, then brush 561 engages segments 571 and 553.Brush 561 continues to make contact with segments 571 and 553 as long asthe aforementioned brushes make contact with any of the last ten sendersof the upper part of the bank. When any of these ten senders is selectedfor cooperation with a calling line finder, the subsequent release ofthe link will not cause the sender selector elevator to remain on theterminals of the sender just used but causes it to return to normal.When relay 527 releases as the result of the removal of ground onconductor 519 by the return of the link finder shaft to normal, relay525 is also released and a circuit is closed from ground on the bottomouter back contacts of relay 527, conductor 556, commutator segment 571,brush 561, commutator segment 553, conductor 558. bottom contacts ofrelay 525, winding of relay 505 to battery. Relay 505 closes an obviouscircuit for'down drive magnet 529 which returns the sender selectorelevator to normal.

The object of the foregoing operation when connection is established toone of the upper ten senders is to eliminate, on subsequent calls, thelong delay incident to returning the sender selector elevator to normalshould it be found that the sender to which the elevtacr is thenconnected, as well as those above it, are all busy. Under suchcircumstances the subscriber would have to wait until the elevator isreturned before hunting over the remaining larger number of senderscould of the previous call so that the sender selector can startimmediately from the bottom of the bank to hunt for a sender on thesubsequent call.

It will be recalled that when the line finder seized the calling line.relay 205 released and switch 300 was left in position 3. In thisposition a path i s closed from battery through relay 203. conductor 244to the left upper contact of cam 310. Another path is likewise closedfrom battery through upper wind ing of relay 200. conductor 256 to theleft lower contact and cam 310. These two paths in parallel are extendedover cam 310 and right lower contact thereof, conductor 539 7 of relay603 to ground. Relay 200 is marginal and does not operate. Relay 203operates but is ineffective at this time. Relay 603 operates and closesan obvious circuit for relay 607, which, in turn, closes a circuit forrelay 609. Relay 609 closes a circuit for relay 610.

' As soon as sequence switch 300 arrived in position 3, it prepared thepulsing circuit for receiving dial impulses. This circuit may be tracedfrom battery through the left winding of relay 600, conductor 549 andassociated terminal, brush 565, No. 5 contacts of re lay 525, conductor532, brush 565, conductor 541 and associated terminal, left contact oicam 245, winding of relay 201, bottom inner contacts of relay 200,conductor 218 and associated brush, conductor 113 and associatedterminal, over the subscribers loop, conductor 107 and associatedterminal, conductor 217 and associated brush, top contact of relay 200,cam 225 and left contact thereof, con ductor 540 and associatedterminal, brush 566, conductor 533, No. 4 contacts of relay 525, brush566, conductor 548 and associated terminal to ground on the contacts ofrelay 613. Relay 600 operates, in turn operates relay 618 in a wellknown manner, relay 618 operating relay 619. A circuit is thereforeclosed from the source of tone 616 to the right winding of relay 600,switch 620 in its normal position, front contact of relay 619, contactof cam 614, to ground on the right outer front contacts of relay 609.This tone is transmitted to the calling subscriber to inform him thatthe sender is ready to receive impulses which he may then send out bymanipulating his dial.

The registers of the sender are positit "16d, translator 630 operatesand selections made under the control of the sender as described in theabove identified patent to O. H. Kopp. The district selector iscontrolled through closure of the fundamental circuit with districtsequence switch 300 in positions 4 and 6 in the known manner after thesubscriber has dialed the office code and sequence switch 300 isadvanced to position 8 to move the district selector brushes over thedistrict bank terminals for the selection of idle trunk in the wanteddirection. Thereafter the district selector sequence switch 300 isadvanced to position 10 in the known manner to await the positioning ofthe selectors in the called oflice. When the connection has be i up tothe called subscribers line, rever. battery from the incoming selectorop ates relays 622 and 623 in the usual manner. lays in turn cause theoperation of relay 631 and relays 624 and 604. With these relaysoperated the sender is then advanced to position for making talkingselections and the district selector sequence switch 300 is advanced outof position 10 towards position 16. In passing through positions 12 1-and 14 ground impulses are sent back to the sender in the usual mannerfor talking selection and when the sender is cult is opened and relay508 releases.

satisfied, the fundamental circuit is opened and relay 20$,which forms apart of the fundamental circuit, is released. The district selectorsequence switch 300 is thus advanced to either position lil, 141 or 15,depending upon which talking selection is made. lVhen switch 300 isadvanced beyond position 11, the parallel circuit of relays 200 and 203is broken at cam 310. Relay 203, however, is not released as a lockingcircuit is provided from ground on cam 312 and left upper contact.thereof, conductor 325, contacts of relay 201, which relay is heldenergized over the calling suhscr bers loop in the known man ner,resistance 2-1-6, winning of relay 203 to battery. This ground, however,is also connectedby way of cam 312and the left lower contact thereoftoconductor 539 and thence over an aforetraced circuit to the windings ofrelay 603. Relay 603 now releases, in turn releasing relay 607. Therelease of relay 607 does not release relay 609 since this relay islocked on its own right inner contacts. A circuit is therefore closed.from ground over the left inner back contacts of relay 607, leftcontacts of relay 609, to the winding of relay 605, which operates andlocks over its left inner front contacts to conductor 547 and to groundas above traced.

After talking selections are completed relay 615 operates, connectingbattery through resistance 632 to conductor 547 in parallel with thewinding of relay 60 5.. Relay 508 of the link circuit is held over brush520 in series with the right winding of relay 207 in the districtselector through position 12 of sequence switch 300. lVhen the switch300 advances beyond position 12 d ring the talk ing selection thecircuit for relays 508 and 207 is transferred to ground on the rightouter contacts of relay 206 over conductor 253, which relay is operatedfor talking selection. l hen relay 206 releases on the termination oftalking selection this locking cir- The release of this relay closes acircuit for relay 516 from ground on commutator segment 51S, conductor519, bottom back contacts of relay 508, winding of relay 516 to battery.Relay 516 operates and closes'a circuit for relay 523 from batterythrough winding of relay 523, bottom inner contacts of relay 506, No. 3contactsof relay 516, bottom back contacts of relay 508, conductor519,commutater segment 518 to ground on brush 517. Relay 528 operates,closes obvious circuit for down drive magnet 566 and the link finderelevator is restored to normal disconnecting the sender from the linefinder and causing the release therein of relay 605 which in turnrestores the sender to normal in the known manner. When the elevatorreaches the normal position, ground is disconnected the down drivemagnet. Since relay 207 is still locked over its left winding, startingground is not connected to conductor 110. With the restoration of thelink finder elevator to normal, relay 527 islikewise released, inturnreleasing relay 525 which disconnects'the sender selector brushesfrom the sender.

Inasmuch as thepresent invention is not concerned with the specificoperation of the district selector shown in Fig. 2 and Fig. 3, beyondwhat has heretofore been described or the operation of selectors tofurther eX- tend the calling line and the functionsof the sender tocontrol selections no detailed description thereof has been given hereinas such op rations are completely described in Patent 1,567,072, grantedto WV. H. Matthies on December 29, 1925 as well as the. abovementionedpatent to O. H. Kopp. The selector circuits employed in theestablishment of the talking connection are released following thetermination of the conversation in the manner fully setforth in thepatents hereinbefore referred to.

What is claimed is? 1. In a telephone exchange system, line findersarranged in groups, major sub-groups and minor sub-groups, apluralityofsuperimposed contact banksdivided into a plurality of sections, a'group'of links arranged for cooperation with each of said sections, thevarious major sub-groups of line finders being so arranged in thevarious sections that a different major subgroup appears-as first choiceto the link group in each of said sections and means responsive to theoperation of a line finder for starting an idle'one of the links whichserve a section of said banks in which the minor sub-group containingthe contact set of-the said operated line finder appears as firstchoice. i

2. In a telephone exchange system, line finders arranged in groups,major subgroups and minor sub-groups, aplurality of-superimposed contactbanks divided into a plurality of sections, a group of links arrangedfor cooperation with each of'said sections, the various major subgroupsof line finders being so arranged in the various sections that adifferent major sub-group appears as first choice in eachof saidsections, means responsive to the operation of a line finder for starting an idle oneof the links which serves a section of said bankscontaining the contact set of the operated line finderappearing as firstchoice, and means operated when all links-having first choice access arebusy for starting an idle one of the links of the next succeeding groupbefore which said operated finder appears as second choice.

3. In a telephone exchange system, line finders arranged in groups,major sub-groups and minor sub-groups, a plurality of superimposedcontact banks divided into a plural-

